Abstract

Harnessing short-lived photoexcited electron-hole pairs in metal nanostructures has the potential to define a new phase of optoelectronics, enabling control of athermal mechanisms for light harvesting, photodetection and photocatalysis. To date, however, the spatiotemporal dynamics and transport of these photoexcited carriers have been only qualitatively characterized. Plasmon excitation has been widely viewed as an efficient mechanism for generating non-thermal hot carriers. Despite numerous experiments, conclusive evidence elucidating and quantifying the full dynamics of hot carrier generation, transport, and injection has not been reported. Here, we combine experimental measurements with coupled first-principles electronic structure theory and Boltzmann transport calculations to provide unprecedented insight into the internal quantum efficiency, and hence internal physics, of hot carriers in photoexcited gold (Au)-gallium nitride (GaN) nanostructures. Our results indicate that photoexcited electrons generated in 20 nm-thick Au nanostructures impinge ballistically on the Au-GaN interface. This discovery suggests that the energy of hot carriers could be harnessed from metal nanostructures without substantial losses via thermalization. Measurements and calculations also reveal the important role of metal band structure in hot carrier generation at energies above the interband threshold of the plasmonic nanoantenna. Taken together, our results advance the understanding of excited carrier dynamics in realistically-scaled metallic nanostructures and lay the foundations for the design of new optoelectronic devices that operate in the ballistic regime.

This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award No. DE-SC0004993. R.S., A.S.J., and P.N. acknowledge support from NG NEXT at Northrop Grumman Corporation. Calculations in this work used
the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office
of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. A.D. and H.A.A. acknowledge
support from the Air Force Office of Scientific Research under grant FA9550-16-1-0019. G.T. acknowledges support from
the Swiss National Science Foundation through the Early Postdoc Mobility Fellowship, grant n. P2EZP2_159101. P.N.
acknowledges support from the Harvard University Center for the Environment (HUCE). A.S.J. thanks the UK Marshall
Commission and the US Goldwater Scholarship for financial support. AJW acknowledges support from the National
Science Foundation (NSF) under Award No. 2016217021.